1. Field of the Invention
The present invention relates to an active matrix type reflective liquid crystal display device used for a computer, audiovisual (AV) equipment, etc.
2. Description of the Related Art
In general, in an electrode structure of a liquid crystal display device having switching elements, storage capacitance electrodes forming storage capacitance are provided in addition to pixel driving electrodes for driving a liquid crystal layer. In the case where pixel electrodes are provided via an interlayer insulator on a substrate, since the thickness of the interlayer insulator is large, storage capacitance electrodes are provided under the interlayer insulator.
FIGS. 24A and 24B are diagrams showing an exemplary structure of the above-mentioned liquid crystal display device. FIG. 24A is a plan view thereof, and FIG. 24B is a cross-sectional view thereof taken along a line A-Axe2x80x2 in FIG. 24A. The liquid crystal-display device shown in these figures is described in Japanese Laid-open Publication No. 9-152625.
Referring to FIGS. 24A and 24B, a thin film transistor (TFT) 24 is provided on a substrate 31. An underlying electrode 25 is formed so as to be connected to a drain electrode 36b of the TFT 24. In the case where the underlying electrode 25 is integrally formed with the drain electrode 36b, the underlying electrode 25 and the drain electrode 36b may be collectively called a drain electrode. An interlayer insulator 38 is formed so as to cover the underlying electrode 25 and the drain electrode 36b. A pixel electrode 21 formed on the interlayer insulator 38 is electrically connected to the underlying electrode 25 or the drain electrode 36b through a contact hole 26. Furthermore, the underlying electrode 25 extends to a central portion of a pixel region 50. A storage capacitance electrode 25a at an end of the underlying electrode 25 is opposed to a storage capacitance line 27. The storage capacitance line 27 is formed under a gate insulating film 33. The gate insulating film 33 covers a gate electrode 32 forming a part of the TFT 24. Storage capacitance is formed in a portion where the storage capacitance line 27 and the storage capacitance electrode 25a are opposed to each other interposing the gate insulating film 33 therebetween. The underlying electrode 25 for forming storage capacitance is provided in a narrow width, except for a portion other than the storage capacitance electrode 25a. 
However, in the case where the underlying electrode 25 under the pixel electrode 21 is locally formed in the pixel region 50 (i.e., the underlying electrode 25 is formed in a shape not corresponding to that of the pixel electrode 21), a portion of the interlayer insulator 38 where the underlying electrode 25 is present and a portion of the interlayer insulator 38 where the underlying electrode 25 is not present may be affected in a different manner from each other during a production process. Because of this, the thickness of the resultant interlayer insulator 38 becomes nonuniform, and the pixel electrode 21 may not be formed in a desired shape on the interlayer insulator 38. In the case of a reflective liquid crystal display device, reflective electrodes (pixel electrodes) are not uniformly formed, and irregularities of reflection characteristics are observed. Particularly, in the case where the interlayer insulator 38 is formed so as to have uneven surface by heat treatment during a production process, the difference in thermal conductivity between a portion of the interlayer insulator 38 where the underlying electrode 25 is present and a portion of the interlayer insulator 38 where the underlying electrode 25 is not present is reflected onto the shape of the upper surface of the interlayer insulator 38. This results in display unevenness.
In addition to the above-mentioned problem, there is a possibility that defects are caused in an active matrix substrate having switching elements during a production process. This results in display defects such as line defects, bright points, and flickering. Therefore, in order to enhance production yield, various defect repair techniques have been developed. Mass-production efficiency has been improved by using one of the defect repair techniques or the combination of several kinds of defect repair techniques.
FIG. 25 illustrates the first prior art defect repair technique utilizing a structure of storage capacitance (disclosed in Japanese Publication for Opposition No. No. 4-73569).
The above-mentioned technique is carried out as follows. A MOS-type transistor 208 is turned on by a currently selected scanning signal line 202. A pixel electrode 206 is charged with a signal of a data signal line 201. At this time, as shown in a circuit configuration diagram of FIG. 26, a liquid crystal capacitance 204 and a storage capacitance 205 are charged with a signal of the data signal line 201 through the MOS-type transistor 208. Thus, in the case where the liquid crystal capacitance 204 becomes small and influence of parasitic capacitance becomes large due to miniaturization of a pixel, the liquid crystal capacitance 204 can be compensated by the storage capacitance 205. The liquid crystal layer capacitance 204 is formed between the pixel electrode 206 and a counter electrode (not shown) on a counter substrate which is opposed to the pixel electrode 206 via a liquid crystal layer. On the other hand, the storage capacitance 205 is formed between the pixel electrode 206 and a non-selected scanning signal line 202 interposing a gate insulating film therebetween.
In the case-where pinholes are generated in the gate insulating film in the storage capacitance 205 during a production process, since the pixel electrode 206 overlaps the non-selected scanning signal line 202 via the gate insulating film in the storage capacitance 205, the pixel electrode 206 is electrically connected to the non-selected scanning signal line 202. Therefore, a data signal is not appropriately applied to the pixel electrode 206. In this case, the following extreme defects are caused: the pixel remains in a light-up state, or the pixel does not light up. In order to prevent such defects, the storage capacitance electrode 207 (where the pixel electrode 206 overlaps the scanning signal line 202) could be insulated from the other pixel electrode portions by a slit 210 except for a portion of the storage capacitance electrode 207. Because of this, even in the case where pinholes are generated, a constricted portion provided by the slit 210 is cut with a laser beam during the later correcting step, whereby the storage capacitance electrode 207 is completely insulated from the other pixel electrode portions. Thus, the above-mentioned extreme defects are eliminated, and the pixel is controlled with a signal from the data signal line (i.e., the pixel is driven with a data signal without storage capacitance). As a result, improved effects are obtained.
Referring to FIGS. 27A and 27B, the second prior art technique will be described. In the second prior art technique, line defects, which are caused when a data signal line 201 and a scanning signal line 202 are short-circuited in an MOS-type transistor 208, are corrected (disclosed in Japanese Publication for Opposition No. 3-55985)
A gate electrode 220 branched from the scanning signal line 202 is cut with a laser beam from the scanning signal line 202 along a broken line shown in FIG. 27A. Thereafter, as shown in FIG. 27B, a laser beam is radiated in directions of arrows xcex1 and xcex2 from above a substrate. Because of this, a source electrode 221 and a drain electrode 222 are short-circuited via the cut gate electrode 220. As a result, an average voltage of a data signal is applied to the pixel electrode 206, whereby the presence of defects may be made unnoticeable.
However, the above-mentioned repair techniques are predicted for a transmission-type liquid crystal display device, and have the following problems.
Firstly, since laser irradiation is conducted in the vicinity of a TFT element, the other films are adversely affected, and repair may not be successful.
Secondly, regarding a pixel without any defect, a voltage which is slightly decreased by a TFT element structure (i.e., parasitic capacitance formed in a portion where a gate electrode overlaps a drain electrode) is applied to a liquid crystal layer corresponding to the pixel without any defect. In contrast, regarding a defective pixel, the source electrode 221 and the drain electrode 222 are short-circuited even after repair of defects, so that a voltage drop caused by the parasitic capacitance is not generated, and a data signal is directly applied to a liquid crystal layer corresponding to the defective pixel. Therefore, particularly in a normally black mode, and a liquid crystal display mode having steep threshold characteristics, even when the identical signal is applied to the pixel without any defects or to the pixel with its defects corrected, optical characteristics of a liquid crystal layer are substantially varied therebetween, depending upon the above-mentioned voltage drop. Accordingly, no substantial effect of the repair of defects is obtained.
A reflective liquid crystal display device is provided in which, on one of a pair of substrates opposed to each other with a liquid crystal layer interposed therebetween, a plurality of data signal lines for supplying a data signal and a plurality of scanning signal lines for supplying a scanning signal are provided so as to cross each other, a plurality of thin film transistors are provided so as to be electrically connected to the plurality of data signal lines and the plurality of scanning signal lines, a plurality of reflective pixel electrodes are provided on an interlayer insulator formed so as to cover at least a part of the plurality of data signal lines, the plurality of scanning signal lines, or the plurality of thin film transistors, and the plurality of reflective pixel electrodes are electrically connected to drain electrodes of the plurality of thin film transistors through contact holes provided in the interlayer insulator, wherein an underlying film is disposed in contact with the interlayer insulator, the underlying film uniformizing heat conduction and/or light transmittance between the interlayer insulator and the one of the substrates and/or contact properties of the interlayer insulator with respect to the one of the substrates in a region where at least one of the plurality of reflective pixel electrodes is formed.
In one embodiment of the present invention, the underlying film is formed of the drain electrode of the thin film transistor, the drain electrode has a first portion to which a voltage is directly applied from a drain of the thin film transistor, a second portion connected to the first portion and including an electric connecting portion for electrical connection to the reflective pixel electrode via the contact hole, and a third portion to which the voltage is applied via the second portion, and at least one constricted portion of the drain electrode having a narrow width is provided between the first portion and the second portion and between the second portion and the third portion.
In another embodiment of the present invention, the constricted portions are provided so as to be close to the electric connecting portion.
In another embodiment of the present invention, the underlying film is formed of the drain electrode of the thin film transistor, and a ratio of an area of the drain electrode to an area of the reflective pixel electrode is 50% to 95%.
In another embodiment of the present invention, the ratio of an area of the drain electrode to an area of the reflective pixel electrode is 60% to 95%.
In another embodiment of the present invention, the underlying film is formed of the drain electrode of the thin film transistor and at least one island-shaped thin film electrically insulated from the drain electrode, and a ratio of a total area of the drain electrode and the at least one island-shaped thin film to an area of the reflective pixel electrode is 40% to 90%.
In another embodiment of the present invention, the total area of the drain electrode and the at least one island-shaped thin film to an area of the reflective pixel electrode is 50% to 90%.
In another embodiment of the present invention, the drain electrode and the at least one island-shaped thin film are made of the same material.
In another embodiment of the present invention, the underlying film formed in contact with the interlayer insulator is provided so as to correspond to a shape of the reflective pixel electrode to be formed on the interlayer insulator.
In another embodiment of the present invention, at least part of the underlying film is a part of an electrode forming storage capacitance.
Hereinafter, the function of the present invention will be described.
According to the present invention, an underlying film is formed under an interlayer insulator so as to be in contact therewith. The underlying film is formed for the purpose of uniformizing the heat conduction and/or light transmittance between the interlayer insulator and the substrate and/or the contact properties of the interlayer insulator with respect to the substrate, in a region where at least one of a plurality of reflective pixel electrodes is formed. The underlying film can prevent or suppress partial difference in thermal conductivity in the interlayer insulator during the step of forming the interlayer insulator. Because of this, during production of the interlayer insulator on the underlying film, each part is subjected to uniform conditions during each step, and the upper surface of the interlayer insulator can be prescribed to be desirably uneven. This enables a reflective pixel electrode provided on the interlayer insulator to have a desired uneven shape. This prevents reflection characteristics from having irregularities. Furthermore, for example, even in the case where unevenness is formed on the interlayer insulator made of a photosensitive material only by UV-irradiation, the intensity of UV-light and the permeation of a developer are uniformized between a portion of the interlayer insulator under which the underlying film is present and a portion of the interlayer insulator under which the underlying film is not present. Therefore, the reflective pixel electrode becomes unlikely to be varied by the underlying film. Furthermore, even in the case where the upper surface of the interlayer insulator is prescribed to be flat, since the underlying film is formed, the intensity of UV-light and the permeation of a developer are uniformized between a portion of the interlayer insulator under which the underlying film is present and a portion of the interlayer insulator under which the underlying film is not present. Furthermore, the, height at these portions can be aligned, and the thickness of the interlayer insulator in these portions can be made uniform. Therefore, it becomes unlikely that the shape of the reflective pixel electrode will be varied by the underlying film. Furthermore, in the reflective liquid crystal display device of the present invention, the reflective pixel electrode is made of metal or the like, so that the structure of the underlying portion of the pixel electrode does not affect a display. Thus, the inventors of the present invention found the structure of the underlying portion of the pixel electrode which is peculiar to the reflective liquid crystal display device and convenient for correcting a defect.
More specifically, a defect is corrected as follows. As described in the prior art section, a gate electrode is cut from a scanning signal line in accordance with a short-circuited position between the data signal line or the scanning signal line and the drain electrode, and the source electrode and the drain electrode are short-circuited or a predetermined constricted portion is cut in accordance with the short-circuited position.
At this time, when the constricted portion is provided close to an electric connecting portion, a portion close to the electric connecting portion can be cut. Therefore, a short-circuited portion can be prevented from remaining between the constricted portion and the electric connecting portion after repair of a defect. Furthermore, the constricted portion is preferably at least about 6 xcexcm from the electric connecting portion. This is because a positional shift of a contact hole caused during formation thereof is about 3 xcexcm, and a diameter of the area of the interlayer insulator between the drain electrode and the pixel electrode which is irradiated with a laser beam is about 3 xcexcm.
Furthermore, when the underlying film is formed only of the drain electrode, the underlying film which uniformizes the effect on the interlayer insulator can be obtained, without increasing the number of steps of forming a new film. When the ratio of the area of the drain electrode to the area of the reflective pixel electrode is prescribed to be about 50% to about 95%, preferably about 60% to about 95%, it becomes unlikely that a short-circuit will occur between the drain electrode and the surrounding data signal lines.
Furthermore, when the underlying film is formed of the drain electrode and at least one island-shaped thin film electrically insulated from the drain electrode, the shape and arrangement of the underlying film can be flexibly prescribed. Furthermore, when the underlying film is formed in such a manner as to include the drain electrode and the island-shaped thin film electrically insulated therefrom, even in the case where a short-circuit is caused between the drain electrode and the surrounding data signal lines, it is unlikely for the pixel electrode to be affected. Thus, the number of steps of correcting a defective pixel can be decreased, whereby a percentage of satisfactory products can be enhanced. When the total area of the drain electrode and the island-shaped thin film is prescribed to be about 40% to about 90%, more preferably about 50% to about 90% of the area of the reflective pixel electrode, the probability of generation of defective pixels can be decreased without fail.
Furthermore, when the drain electrode and the island-shaped thin film are made of the same material, the drain electrode and the island-shaped thin film can be formed during the same step.
Furthermore, when the underlying film provided in contact with the interlayer insulator is formed so as to correspond to the shape of the reflective pixel electrode on the interlayer insulator, the effect on the interlayer insulator can be uniformized with more reliability.
Even in the case where at least part of the underlying film of the present invention is provided as a part of an electrode which forms storage capacitance, the effect on the interlayer insulator can be uniformized, and large storage capacitance can be secured.
Thus, the invention described herein makes possible the advantages of (1) providing a reflective liquid crystal display device capable of preventing generation of display roughness caused by an underlying electrode locally provided in a pixel region; and (2) providing a reflective liquid crystal display device capable of correcting pixel defects generated during a production process.